In MRAM (magnetic random access memory), a cell's magnetic state is switched (i.e. its direction of magnetization is rotated) by an external magnetic field. This field is generated by a current through a program line on a chip. It is desirable to lower the switching threshold of MRAM cells so that the write current, and thus the chip power, can be reduced. The free layer in an MRAM cell is typically a soft magnetic film having a rectangular or oval shape.
The direction of the magnetization, M, in a magnetic soft film is determined by the lowest energy state of the film. In the sub-micron geometry film, the shape anisotropy Hd dominates over the crystalline anisotropy HK. The direction of the HK is usually called the easy axis direction of the film. Its direction is set by a small magnet in the deposition chamber during film deposition. The direction of the film's magnetization M would normally parallel the largest dimension of the cell to minimize demagnetization effects arising from the shape anisotropy due to its dominance in small geometry film, rather than the direction of the crystalline anisotropy. This is illustrated in FIG. 1 where memory element 11 is a rectangle of length 12 and width 13. As a result the magnetization would parallel the direction of 12. Nonetheless, the direction of crystalline anisotropy, or the direction of the easy axis, still plays a role in the magnitude of the switching field of the cell. The present invention discloses a method and structure that lowers the requirements for the switch field of the soft film. It is applicable to the MRAM cells.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,104,633, Abraham et al. show a MRAM and discuss the easy axis of the free layer. Abraham et al. also show related MRAMs in U.S. Pat. No. 6,072,718 and in U.S. Pat. No. 5,946,228. U.S. Pat. No. 6,259,644B (Tran et al.) shows a MRAM with free and pinned layers in anti-parallel directions while Sun discusses switching thresholds in U.S. Pat. No. 6,256,223 B1.